Hundreds of wildfires burning in Canada are undermining U.S. air quality across the Upper Midwest and Northeast, making skies hazy from Detroit to New York City and even prompting a ground stop at Boston's Logan International Airport due to "low visibility.

The low visibility caused by the smoke haze reached a safety threshold that forced an operational pause and delayed flights into and out of BOS. ABC News reports that at least 742 different fires are burning in Canada now, with the 15 latest burns sparking as recently as Sunday.

As Canada grapples with one of its worst wildfire seasons on record, smoke from the blazes have prompted air quality alerts across the U.S.